Power transmission [120 marks] t I 1. The graph shows the variation with time of the current in the primary coil of an ideal transformer. [1 mark] The number of turns in the primary coil is 100 and the number of turns in the secondary coil is 200. Which graph shows the variation with time of the current in the secondary coil? 2. The diagram shows a diode bridge rectification circuit and a load resistor. [1 mark] The input is a sinusoidal signal. Which of the following circuits will produce the most smoothed output signal? The graph shows the power dissipated in a resistor of 100 Ω when connected to an alternating current (ac) power supply of root 3. [1 mark] mean square voltage (Vrms) 60 V. What are the frequency of the ac power supply and the average power dissipated in the resistor? A capacitor consists of two parallel square plates separated by a vacuum. The plates are 2.5 cm × 2.5 cm squares. The capacitance of the capacitor is 4.3 pF. Calculate the distance between the plates. 4a. [1 mark] 4b. The capacitor is connected to a 16 V cell as shown. [2 marks] Calculate the magnitude and the sign of the charge on plate A when the capacitor is fully charged. ε ε 4c. The capacitor is fully charged and the space between the plates is then filled with a dielectric of permittivity = 3.0 0. [2 marks] Explain whether the magnitude of the charge on plate A increases, decreases or stays constant. In a different circuit, a transformer is connected to an alternating current (ac) supply. 4d. [3 marks] The transformer has 100 turns in the primary coil and 1200 turns in the secondary coil. The peak value of the voltage of the ac supply is 220 V. Determine the root mean square (rms) value of the output voltage. Describe the use of transformers in electrical power distribution. 4e. [3 marks] number of primary turns The ratio for a transformer is 2.5. 5. number of secondary turns [1 mark] The primary coil of the transformer draws a current of 0.25 A from a 200 V alternating current (ac) supply. The current in the secondary coil is 0.5 A. What is the efficiency of the transformer? A. 20 % B. 50 % C. 80 % D. 100 % E T 6. An alternating current (ac) generator produces a peak emf 0 and periodic time . What are the peak emf and periodic time when [1 mark] the frequency of rotation is doubled? A direct current (dc) of 5A dissipates a power P in a resistor. Which peak value of the alternating current (ac) will dissipate an 7. [1 mark] average power P in the same resistor? A. 5A B. 5 A 2 C. 5 A √2 D. 5√2A State Faraday’s law of induction. 8a. [2 marks] The diagram shows a sketch of an ideal step-down transformer. The number of turns in the primary coil is 1800 and that in the secondary coil is 90. Explain, using Faraday’s law of induction, how the transformer steps down the voltage. 8b. [4 marks] The input voltage is 240 V. Calculate the output voltage. 8c. [2 marks] Outline how energy losses are reduced in the core of a practical transformer. 8d. [2 marks] Step-up transformers are used in power stations to increase the voltage at which the electricity is transmitted. Explain why this is 8e. [2 marks] done. A cable consisting of many copper wires is used to transfer electrical energy from an alternating current (ac) generator to an electrical load. The copper wires are protected by an insulator. The cable consists of 32 copper wires each of length 35 km. Each wire has a resistance of 64 Ω. The cable is connected to the ac generator which has an output power of 110 MW when the peak potential difference is 150 kV. The resistivity of copper is 1.7 x 10–8 Ω m. output power = 110 MW Calculate the radius of each . 9a. wire [2 marks] Calculate the peak current in the . 9b. cable [1 mark] Determine the power dissipated in the cable per unit length. 9c. [3 marks] To ensure that the power supply cannot be interrupted, two identical cables are connected in parallel. Calculate the root mean square (rms) current in each cable. 9d. [1 mark] The two cables in part (c) are suspended a constant distance apart. Explain how the magnetic forces acting between the cables 9e. [2 marks] vary during the course of one cycle of the alternating current (ac). The energy output of the ac generator is at a much lower voltage than the 150 kV used for transmission. A step-up transformer is used between the generator and the cables. Suggest the advantage of using a step-up transformer in this way. 9f. [2 marks] The use of alternating current (ac) in a transformer gives rise to energy losses. State how eddy current loss is minimized in the 9g. [1 mark] transformer. Which of the following reduces the energy losses in a transformer? 10. [1 mark] A. Using thinner wires for the windings. B. Using a solid core instead of a laminated core. C. Using a core made of steel instead of iron. D. Linking more flux from the primary to the secondary core. The secondary coil of an alternating current (ac) transformer is connected to two diodes as shown. 11. [1 mark] Which graph shows the variation with time of the potential difference VXY between X and Y? The following data are available for a natural gas power station that has a high efficiency. Rate of consumption of natural gas = 14.6 kg s–1 Specific energy of natural gas = 55.5 MJ kg–1 Efficiency of electrical power generation = 59.0 % Mass of CO2 generated per kg of natural gas = 2.75 kg One year = 3.16 × 107 s 12a. Electrical power output is produced by several alternating current (ac) generators which use transformers to deliver energy to the [4 marks] national electricity grid. The following data are available. Root mean square (rms) values are given. ac generator output voltage to a transformer = 25 kV ac generator output current to a transformer = 3.9 kA Transformer output voltage to the grid = 330 kV Transformer efficiency = 96% (i) Calculate the current output by the transformer to the grid. Give your answer to an appropriate number of significant figures. (ii) Electrical energy is often delivered across large distances at 330 kV. Identify the main advantage of using this very high potential difference. 12b. In an alternating current (ac) generator, a square coil ABCD rotates in a magnetic field. [5 marks] The ends of the coil are connected to slip rings and brushes. The plane of the coil is shown at the instant when it is parallel to the magnetic field. Only one coil is shown for clarity. The following data are available. Dimensions of the coil = 8.5 cm×8.5 cm Number of turns on the coil = 80 Speed of edge AB = 2.0 ms–1 Uniform magnetic field strength = 0.34 T (i) Explain, with reference to the diagram, how the rotation of the generator produces an electromotive force (emf ) between the brushes. (ii) Calculate, for the position in the diagram, the magnitude of the instantaneous emf generated by a single wire between A and B of the coil. (iii) Hence, calculate the total instantaneous peak emf between the brushes. V P¯ 13. An alternating current (ac) power supply generates an emf with peak amplitude 0 and delivers an average power . What is the [1 mark] root mean square (rms) current delivered by the supply? P¯ A. 2V0 P¯ B. √2V0 √2P¯ C. V0 2P¯ D. V0 A full-wave diode rectification circuit is modified with the addition of a capacitor in parallel with the load resistor. The circuit is used to 14. [1 mark] rectify a sinusoidal signal of period 6.3ms. Which graph shows how the potential difference V across the load varies with time? 15. Which of the following experiments provides evidence for the existence of matter waves? [1 mark] A. Scattering of alpha particles B. Electron diffraction C. Gamma decay D. Photoelectric effect 16. An alternating current is sinusoidal and has a maximum value of 1.5 A. What is the approximate value of the root mean squared [1 mark] (rms) current? A. 2.3 A B. 1.5 A C. 1.0 A D. 0.75 A Part 2 Power transmissions The diagram shows the main features of an ideal transformer whose primary coil is connected to a source of alternating current (ac) voltage. 17a. Outline, with reference to electromagnetic induction, how a voltage is induced across the secondary coil. [3 marks] 17b. The primary coil has 25 turns and is connected to an alternating supply with an input voltage of root mean squared (rms) value 12 [2 marks] V. The secondary coil has 80 turns and is not connected to an external circuit. Determine the peak voltage induced across the secondary coil. A different transformer is used to transmit power to a small town. The transmission cables from the power station to the transformer have a total resistance of 4.0 Ω. The transformer is 90% efficient and steps down the voltage to 120 V. At the time of maximum power demand the effective resistance of the town and of the cables from the transformer to the town is 60 mΩ. 17c. Calculate the current in the cables connected to the town [1 mark] Calculate the power supplied to the transformer.